This is a false positive due to a minor bug in the rule itself. When trying to figure out if a class re-implements IDisposable (after figuring out that there's a base class implementation that could be overridden), it only looks at whether the class' interfaces include IDisposable. Unfortunately, the interface list that shows up in the assembly metadata includes the "exploded" list of interfaces, including any interfaces inherited via interfaces that the class explicitly implements in the original C# code. This means that FxCop is seeing a declaration that looks like the following for your Functionality class:

Given this metadata representation, the ImplementIDisposableCorrectly rule should be a bit more intelligent about how it attempts to determine whether the class is actually re-implementing IDisposable (for example, by looking for an explicit Dispose() implementation if the base class has an overridable Dispose(bool)). However, given that the rule doesn't do this, your best approach is to suppress the false positives.

BTW, I would recommend seriously considering using SuppressMessageAttribute for suppressing the false positives instead of your current conditional compilation approach. e.g.: